Various audio devices incorporate active noise reduction (ANR) features, also known as active noise control or cancellation (ANC), in which one or more microphones detect sound, such as exterior acoustics captured by a feedforward microphone or interior acoustics captured by a feedback microphone. Signals from a feedforward microphone and/or a feedback microphone are processed to provide anti-noise signals to be fed to an acoustic transducer (e.g., a speaker, driver) to counteract noise that may otherwise be heard by a user. Feedback microphones pick up acoustic signals produced by the driver, and thereby form a closed loop system that could become unstable at times or under certain conditions. Various audio systems that may provide feedback noise reduction include, for example, headphones, earphones, headsets and other portable or personal audio devices, as well as automotive systems to reduce or remove engine and/or road noise, office or environmental acoustic systems, and others. In various situations it is therefore desirable to detect when a condition of feedback instability exists.